We propose a new technique for the generation of entangled-photon beams with high flux by the use of a parametric process, or simultaneous parametric processes, in conjunction with simultaneous laser action from an active medium and higher-harmonic generation in an optical cavity. Laser action can arise via the usual mechanisms associated with a pumped active medium in a cavity or, more generally, by any number of optical processes such as stimulated emission without population inversion. The production of high-flux entangled-photon beams can be achieved by the presence of such an active medium coupled with, or congruent with, various nonlinear media or devices that permit some combination of parametric processes that generate light at new frequencies, both higher and lower than the laser light within the cavity. The generation of such radiation can be continuous wave (cw), or it can be generated in pulsed form by virtue of processes such as gain switching, cavity dumping, Q-switching, mode-locking, combinations thereof, or other means. It can be implemented in a variety of ways, including second- and optical higher-harmonic generation, and so-called type-I or type-II parametric downconversion.
This invention provides a substantial improvement over the current and established process of spontaneous parametric downconversion, which takes place outside a cavity and produces only a low flux of entangled photon pairs. It is to be distinguished from the well-known processes of optical parametric amplification and optical parametric oscillation that take place in a cavity that contains the two modes of the downconverted light. In these latter processes the two members of an entangled photon-pair (twin photons) are emitted from the cavity individually thereby lengthening the entanglement time and diluting the entangled nature of the emitted photon pairs or clusters. In the technique proposed herein, the entangled-photon pairs maintain their close coordination in time, space, momentum, energy, and/or polarization after exiting from the device.
Ideal spontaneous parametric downconversion splits each pump-beam photon into twin daughter photons that are emitted simultaneously. Since energy and momentum are conserved in the splitting process, the daughter photons share the energy and momentum of the mother. This entangles the directions of the two daughters so that the emission of one photon in a given direction is associated with a certain simultaneous emission of a twin photon in a matching direction. The twins may have the same frequency (wavelength or color), in which case they are identical (or degenerate); or differ in frequency (wavelength or color), in which case they are in a sense fraternal (or nondegenerate). The downconverted pairs generated may emerge in the same direction (colinearly) or in different directions (non-colinearly). The twins may also be entangled in polarization for type-II downconversion. The entanglement persists no matter how far away the photons might be from each other.
In one implementation of the invention, the technique can be used for the generation of entangled photons using simultaneous parametric processes such as second-harmonic generation and spontaneous parametric downconversion in conjunction with laser action in a cavity. The parametric processes can be engendered by making use of a single nonlinear device such as a nonlinear optical crystal that produces both second-harmonic generation and parametric downconversion, or separate nonlinear devices, within the cavity. The high energy density within the cavity allows entangled photons with high flux to be produced in such directions that they exit from the cavity after they are generated. This technique is particularly applicable for noncolinear generation.
In another implementation of the invention, multiple laser cavities are used in conjunction with mirrors that have high reflectance at one wavelength and low reflectance at another wavelength, together with dichroic and/or polarization-sensitive optics and multiple intracavity nonlinear crystals, to achieve strong pumping of the nonlinear parametric downconversion process and high-flux entangled photons to be generated and exit the optical system. This technique is particularly applicable for colinear generation.
In another implementation of the invention, multiple laser cavities may be used in conjunction with mirrors that have high reflectance at one frequency and low reflectance at another frequency, together with an optical element or optical elements that angularly separate the different light frequencies. Examples of such elements are dichroic, prism, and/or grating devices and/or polarization-sensitive optics. Multiple intracavity nonlinear optical crystals may also be used to achieve strong pumping of the nonlinear parametric downconversion process and thereby permit high-flux entangled photons to be generated and exit the optical system. This technique is particularly applicable for colinear generation.